gh-154523: Fix data-race in TextIOWrapper.detach() (#154565)

(cherry picked from commit 8440a6a)

Author: brijkapadia

Lib/test/test_free_threading/test_io.py

@@ -198,3 +198,23 @@ def reset_worker():
                 decoder.reset()
 
         run_concurrently([decode_worker] * 2 + [reset_worker] * 2)
+
+
+class TextIOWrapperTest(TestCase):
+    def test_buffer_detach_race(self):
+        make = lambda: io.TextIOWrapper(io.BytesIO())
+        slot = [make()]
+
+        def reader():
+            for _ in range(1000):
+                try:
+                    slot[0].buffer
+                except ValueError:
+                    pass
+
+        def detacher():
+            for _ in range(1000):
+                slot[0] = make()
+                slot[0].detach()
+
+        run_concurrently([reader, detacher])

Misc/NEWS.d/next/Library/2026-07-23-18-53-48.gh-issue-154523.XUSRBO.rst

@@ -0,0 +1,2 @@
+Fixed data-race when calling :meth:`io.TextIOBase.detach` in
+:term:`free-threaded build`.

Modules/_io/clinic/textio.c.h

@@ -3426,6 +3426,19 @@ _io_TextIOWrapper__CHUNK_SIZE_set_impl(textio *self, PyObject *value)
     return 0;
 }
 
+/*[clinic input]
+@critical_section
+@getter
+_io.TextIOWrapper.buffer
+[clinic start generated code]*/
+
+static PyObject *
+_io_TextIOWrapper_buffer_get_impl(textio *self)
+/*[clinic end generated code: output=d265a34555aa5d4b input=5951cfa148f7350a]*/
+{
+    return Py_XNewRef(buffer_access_safe(self));
+}
+
 static PyMethodDef incrementalnewlinedecoder_methods[] = {
     _IO_INCREMENTALNEWLINEDECODER_DECODE_METHODDEF
     _IO_INCREMENTALNEWLINEDECODER_GETSTATE_METHODDEF
@@ -3484,7 +3497,6 @@ static PyMethodDef textiowrapper_methods[] = {
 
 static PyMemberDef textiowrapper_members[] = {
     {"encoding", _Py_T_OBJECT, offsetof(textio, encoding), Py_READONLY},
-    {"buffer", _Py_T_OBJECT, offsetof(textio, buffer), Py_READONLY},
     {"line_buffering", Py_T_BOOL, offsetof(textio, line_buffering), Py_READONLY},
     {"write_through", Py_T_BOOL, offsetof(textio, write_through), Py_READONLY},
     {"_finalizing", Py_T_BOOL, offsetof(textio, finalizing), 0},
@@ -3499,6 +3511,7 @@ static PyGetSetDef textiowrapper_getset[] = {
     _IO_TEXTIOWRAPPER_NEWLINES_GETSETDEF
     _IO_TEXTIOWRAPPER_ERRORS_GETSETDEF
     _IO_TEXTIOWRAPPER__CHUNK_SIZE_GETSETDEF
+    _IO_TEXTIOWRAPPER_BUFFER_GETSETDEF
     {NULL}
 };